(1) Field of the Invention
The present invention relates to a control circuit for a vehicle levelling system and more particularly to a control circuit for a vehicle levelling system having two or more reference vehicle-height positions and capable of switching over from one to another of such reference positions.
(2) Relevant Art
The present applicant has previously proposed a control circuit for a vehicle levelling system in commonly-assigned U.S. Pat. No. 4,391,452 is issued July 5, 1983. This proposed control circuit includes detecting means disposed in both front and rear portions of a vehicle for outputting a directional signal as a detected signal on the direction of change of the vehicle height and also for outputting a command signal for deciding whether a driving system for vehicle height adjustment is to be operated or not. With such signals provided from the detecting means the operation of the mechanical driving system is controlled so that the vehicle height at the front side and at the rear side of the vehicle is always at a reference level. This control circuit, which is shown in FIG. 5 of the accompanying drawings, will be described in detail hereinbelow with respect to the front portion of a vehicle and with reference to FIG. 5.
A directional signal a.sub.1 indicating whether or not the vehicle height at the front side of a vehicle is within a preset reference range is fed from a detector 51a to a terminal 55a, while a command signal b.sub.1 indicating whether or not the foregoing driving system is to be operated is fed from a detector 51b to a terminal 55b. The signals a.sub.1 and b.sub.1 are suitably combined in a logic circuit (A) to operate an actuator 53 for adjusting the front-side vehicle height and also to operate either a compression means (B) or a decompression means (C) whereby the front-side vehicle height is adjusted to a reference level. The rear-side vehicle height is also adjusted in the same manner; that is, a directional signal a.sub.2 and a command signal b.sub.2 are provided from detectors 52a and 52b, respectively, and on the basis of a predetermined operation of the logic circuit (A) an actuator valve 54, etc. are operated. Actually, by the combination of the above four signals a.sub.1, b.sub.1, a.sub.2 and b.sub.2 there are performed a series of vehicle height adjustments with respect to one reference vehicle-height position.
In the aforesaid vehicle levelling system, two points are provided at the front and rear sides of a vehicle for adjusting the vehicle height, but only one reference vehicle height as the target of adjustment, namely, the vehicle height adjusting position, is provided. However, the vehicle height adjusting position is not limited to one; for example, if a pair of detectors which output a directional signal and a command signal are increased in number and input terminals 55a, 56a, 55b, 56b of the control circuit are switched over selectively by means of a switching device, then it becomes possible to change the vehicle height to other desired positions. However, if a vehicle levelling system having such two or more reference vehicle height positions is constructed on the basis of the foregoing control circuit, the following problems occur when changing a detected signal relating to one reference vehicle height position into a detected signal relating to another reference vehicle height position by the switch-over operation of the aforesaid switching device.
The vehicle levelling system shown in FIG. 5 is constructed so that when the vehicle height is within a reference vehicle height range and thus is stable, a command signal (an OFF signal "1") for stopping the operation of the driving system is input to the terminals 55b and 56b of the control circuit. In case the connection relation is switched over from a detector relating to one reference vechicle height position to a detector relating to another reference vehicle height position, the above OFF signal disappears. In the foregoing vehicle levelling system, however, the control circuit is constructed such that even if the above OFF signal disappears at the terminals 55b and 56b, it continues to exist for a certain period of time by the operation of holding circuits K.sub.1 and K.sub.2. Therefore, even if an ON signal is actually produced at the terminals 55b and 56b, the operation for the vehicle height adjustment is interrupted for a certain period of time as mentioned above. This means that at the time of switching over reference vehicle height positions from one to another, the vehicle height adjusting operation is delayed. As a result, there occurs an observable shift and it is impossible to make an immediate vehicle height adjustment according to various changes, such as changes in the condition of a road surface, just after switching operation.
In the case of a control system wherein the front and rear portions of a vehicle are adjusted separately, the time required for adjusting the vehicle height from one level to another differs according to the weight distribution of the vehicle body, the capacity of a pressure source (e.g. air or oil pressure), etc. But, in general, when the vehicle body is light and the pressure source is of sufficient capacity, the time required for the vehicle height adjustment becomes the shortest. Therefore, in order for the vehicle levelling system to operate immediately and sufficiently at the time of switching over reference vehicle height positions selectively from one to another, it is desired that the OFF signal holding action be interrupted at least during the aforesaid period of time required for the vehicle height adjustment.